The present invention relates to a method of joining metals for use at high temperature, especially superalloy materials. The evolution of advanced alloys for service at high temperature has resulted in the development of a number of alloy systems that are very difficult to fabricate and join. The development of modern nickel-base superalloys has produced compositions for use in modern gas turbine engines which have increasingly higher volume fractions of the gamma prime phase, typically Ni.sub.3 (Al,Ti). The increased aluminum and titanium contents, while significantly increasing high temperature creep strength and rupture life for more demanding engine performance requirements, have reduced the ductility of the alloys and thereby increased their susceptibility to cracking and fissuring during welding.
In addition, the heat associated with welding processes affects the structure of such alloys. Columnar-grained and single crystal articles lose their respective crystallographic orientation during the melting of welding. Thus, many of the advanced superalloys and oriented structures cannot be joined by welding and are considered unweldable. The inability to weld more complex shapes limits design and cost flexibility for a designer attempting to use such materials. The use of mechanical attachment, diffusion bonding, and advanced brazing techniques have been the only fabrication alternatives available in the prior art where welding is not viable. Each of these alternatives has inherent limitations.
The most common methods of welding generate heat in a localized area causing melting and fusion of the metal. Welding processes that may be useful with superalloys are gas-metal arc (GMA), gas-tungsten arc (GTA), plasma arc (PA), or electron beam (EB).
Hot cracking during welding and post-weld heat treatment (strain age cracking) are commonly observed in the nickel-base superalloys due to the aluminum and titanium in the gamma (.gamma.) and gamma prime (.gamma.') strengthening phases. All welded joints produce a reduction in mechanical properties, especially ductility since the structure of solidified weld metal is more segregated. If high electron vacancy elements segregate on solidification of the weld, embrittling precipitates such as sigma (.alpha.) or mu (.mu.) can form. Heat affected zones exposed to high temperatures are susceptible to grain growth, solutioning, and reprecipitation of carbides and other phases leading to property degradation, including corrosion and oxidation resistance. By contrast, solid solution cobalt-base alloys can be readily welded.
Diffusion bonding methods, such as vacuum hot pressing, have been partially successful in joining superalloys, but extensive surface preparation and costly fixturing must be employed. Also, dissimilar alloys can result in undesirable phases or Kirkendall porosity from the long exposures to elevated temperature and pressure.
Brazing methods employ the use of a lower melting point filler alloy. The more advanced methods utilize boron as a melting point depressant which can be diffused away from the joint during post braze thermal treatment. Even the most advanced processes, however, provide less than 80% of parent metal strength, are of limited ductility, and exhibit undesirable low-melting phases (e.g., borides, silicides). These braze joint characteristics typically result in increased sensitivity to cracking, microfissuring, and consequently, to reduced low cycle fatigue endurance.
It is an objective of this invention to overcome the limitations of conventional joining processes by eliminating: the segregation in the the joint, deleterious precipitate phases, the significant reduction in mechanical properties in the joint, and the alteration of grain structure of the articles being joined.
Accordingly, it is an objective of this invention to join relatively unweldable components by the use of a spray cast interlayer at joint surfaces.
Additional objectives and advantages will be set forth in part of the description which follows, and in part, will be obvious from the description, or may be learned from the practice of the invention.